Abstract The analytical models which are used to predict the performance of gas-flow and chemical lasers are enumerated and compared. These analytical models include the currently accepted theory of Lorentz-Gauss line shape factor, the present theory of rectangular line shape factor and the theory presented in ref. . For the idealized configuration of the mixing layer of diffusion chemical lasers, the specific results corresponding to the theories mentioned above are developed in brief. Analysis shows that for the general case in which the transition is simultaneously inhomogeneous broadened and collisionally broadened, particularly for the case in which the inhomogeneous broadening is dominant, the difference among the results of the two theories of line shape factor and the theory of article  are quite significant. The results of the theory of rectangular line shape factor are more precise than those of Lorentz-Gauss line shape factor. And the computing-time required by the theory of rectangular line shape factor is less than that of Lorentz-Gauss line shape factor.